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PRETORIA, South Africa - The prosecutor at Oscar Pistorius' murder trial on Friday challenged the athlete's statements that he was worried about crime before he fatally shot girlfriend Reeva Steenkamp through a closed toilet door in his home.

Pistorius has said he killed Steenkamp by accident after mistaking her for an intruder in the early hours of Feb. 14, 2013. He faces 25 years to life in prison if convicted of premeditated murder.

Prosecutor Gerrie Nel examined the details of the alarm system at Pistorius' house, questioning why the athlete would believe an intruder had broken into his home when he had extensive security measures, including interior and exterior sensors. Pistorius said he activated the sensors on the home alarm system before going to sleep on the night he killed Steenkamp, but feared that building contractors doing work on his house may have moved some of the security beacons.

The prosecutor said Pistorius had not mentioned immediately after the shooting that he had fears that building contractors had removed some of the security beacons, specifically near the bathroom window where he allegedly thought an intruder may have gained access on the night he killed Steenkamp.

Querying why Pistorius had not mentioned those fears earlier, Nel said Pistorius was trying to build a story to explain his fears of an intruder and therefore a mistaken shooting.

"This is the biggest example of you tailoring your evidence," Nel said. Pistorius denied he was fabricating a story.

Pistorius also said he was struggling to give clear testimony because he was tired, prompting the judge to ask him if he was too tired to proceed with a tough cross-examination from the chief prosecutor.

Pistorius was responding to a question from Nel, who pointed to a lack of clarity in the double-amputee runner's testimony about whether he turned off the alarm inside his home on the night of Feb. 14, 2013. Pistorius said he "must have" turned off the alarm, which Nel described as a vague response.

The prosecutor then asked Pistorius, who acknowledged making a mistake in his testimony, if he needed time before continuing with his testimony.